The present invention relates generally to the use and construction of electrical circuits and electrical circuit components. More particularly, the present invention includes a life and temperature corrected bi-directional optical coupler for transferring signals between circuits.
Optical couplers, also referred to as optocouplers are a widely used method for interconnecting different circuits. They are particularly useful when signals need to be transferred between circuits that do not (or cannot) share a common ground. Optocouplers are also useful where DC voltage level translations are required.
As an example, consider the circuit of FIG. 1. In this circuit, a microcontroller is used to control the operation of an electric motor. The electrical output of the microcontroller is insufficient to directly drive the motor. For this reason, a motor controller is positioned between the microcontroller and the electric motor. The microcontroller sends a low current output signal to the motor controller. This signal, typically encoded using pulse width modulation (PWM) tells the motor controller how fast the motor is to run. The motor controller includes a rectifier or other circuit that translates the low current PWM output of the microcontroller into a current sufficient to drive the electric motor. The microcontroller, motor controller and motor share a single power supply and ground.
In practice, the use of a common ground and power supply tends to make the circuit of FIG. 1 highly susceptible to noise-based faults. This is because noise associated with the motor and motor controller is transmitted directly to the microcontroller. Unfortunately, most microcontrollers tend to be susceptible to noise with the result that the entire circuit tends to be failure prone.
FIG. 2 demonstrates how an optocoupler can be used to overcome some of the limitations just described with regard to FIG. 1. In the circuit of FIG. 2, an optocoupler is inserted between the microcontroller and the motor controller. The microcontroller is also given its own power supply and ground. As a result, noise generated by the motor and motor controller does not feed back into the microcontroller. This greatly reduces noise-based faults of the microcontroller.
The addition of the optocoupler gives the circuit of FIG. 2 much greater reliability. Still, there are circumstances where the addition of an optocoupler can have it""s own negative side effects. To illustrate, FIG. 3 shows the circuit of FIG. 2 with the addition of tachometer feedback and an error amplifier. The tachometer feedback signal indicates the actual speed of the electric motor. The error amplifier amplifies the signal output by the motor controller (the desired speed) by the difference between the desired speed and the tachometer feedback signal. The amplification adjusts the electric motor speed so that it matches the desired speed.
Unfortunately, the efficiency of the optocoupler varies both as a function of time and as a function of temperature. This efficiency is know as the current transfer ratio (CTR) and is expressed as the ratio of the collector current to the diode forward current (IC/IF). CTR typically decreases over time and decreases as temperature increases. These changes ultimately mean undesirable changes in motor speed.
As illustrated by this example, there is a need for an optocoupler that can be used in a way that reduces the changes in CTR associated with time and temperature. This need is particularly apparent for the motor controlling circuit of FIG. 3. It should be appreciated, however, that the drawbacks associated with current optocouplers apply in a general fashion to a wide range of circuits and environments.
An embodiment of the present invention provides a bi-directional optical coupler. The bi-directional optical coupler includes a package having a first side and a second side. A forward coupler and a reverse coupler are included in the package. The forward coupler receives DC electric signals at the first side of the package and transmits DC electric signals at the second side of the package. The reverse coupler receives DC electric signals at the second side of the package and transmits DC electric signals at the first side of the package.
The two couplers are selected to have closely matched age and performance characteristics. Inclusion of both couplers within a single package means that they age at the same rate (e.g., one cannot be replaced without the other). The inclusion of two couplers within a single package also means that the two couplers are subject to the same temperatures. This means that the performance of the two couplers remains matched as temperatures change during operation.
Stated differently, the present invention includes an integrated circuit that comprises: 1) a package having an inbound side and an outbound side; 2) an forward coupler included within the package, the forward coupler connected to receive DC electric signals at the first side of the package and to retransmit DC electric signals at the second side of the package; and 3) a reverse coupler included within the package, the reverse coupler connected to receive DC electric signals at the second side of the package and to retransmit DC electric signals at the first side of the package.
Other aspects and advantages of the present invention will become apparent from the following descriptions and accompanying drawings.